I am 61 year old woman. i have bad knees and have been told that replacement of knees is the only solution. what exercise can i do for this ?

Water exercises: One of the best exercises for "bad knees" is to walk in the shallow end of a pool. Aquatic exercises are beneficial because they provide resistance with less weight bearing. Stretching (maintaining flexibility) and range of motion exercises (to prevent loss of movement) are also important. See you doctor or physical therapist for more ideas.

Maybe it is time..: I am sorry to hear that replacement is the only option--this must mean your disease is very advanced, and likely very painful. Biking or aquatic exercises are less stressful on the knees, but when the disease is so advanced , it hurts whenever u bend it. If u were 40, i would say wait, but now 60, maybe a good time to get it done..It may improve your quality of life greatly. Consult doc. Good luck.